1. Field of the Invention
This invention relates to antifriction bearings and more particularly to an improved compact antifriction bearing and journal which cooperate to reduce journal flexing and both journal and bearing end face fretting wear during operation.
2. Description of the Prior Art
Sealed two row tapered roller bearing assemblies preassembled into a self-contained, prelubricated package for mounting onto journals at the ends of axles or shafts are well known. Such bearing assemblies are widely used, for example, as rail car bearings mounted onto journals at the ends of rail car axles, and the present invention will be described with reference to such rail car bearings, if being understood that the bearings may be employed on shaft journals for various uses. In bearings of this type, the two rows of tapered rollers are fitted one into the outer tapered race at each end of a common bearing cup for cooperation with a pair of bearing cones defining the inner races and having an inner diameter dimensioned to provide an interference fit with the shaft journal, with a cylindrical sleeve or spacer positioned between the cones to provide accurate spacing and bearing loading. Seals mounted within each end of the bearing cup provide contact with annular wear rings abutting against the outer ends of the respective bearing cones and retain the component parts in assembled relationship so that the entire structure can be pressed as a unit onto the end of the journal.
In the past, it has been considered critical that the seal wear ring interface be kept as small as practically possible in order to minimize the torque required to turn one component relative to the other and to minimize the interface relative speed. Reducing the diameter of the wear ring contact surface reduced the leverage of the frictional resistance and also reduced both the total contact area and the relative circumferential speed between the seal and wear ring contact area. While highly efficient low torque seals now available have reduced the advantages realized from this design consideration, the minimum diameter wear rings have continued to be used.
In a typical rail car installation, the axle journal is machined with a fillet at its inboard end, and a backing ring machined to accurately fit the contour of the fillet along at least a substantial portion of the axial length of the fillet engages and positions the inboard end of the wear ring and thereby accurately locates the bearing on the journal. An end cap mounted on the end of the axle by cap screws threaded into bores in the axle engages the outboard wear ring and applies an axial compressive load to the assembly between the fillet and the end cap and a corresponding axial tensile load to the journal.
Since an interference fit is provided between the inner surfaces of the wear rings and bearing cones and the outer surface of the axle journal, the bearing assembly must be installed on the journal by a pressing operation. Sufficient load is applied during this operation to provide a predetermined seating force against the backing ring and fillet, and upon removal of the seating force, a residual load is maintained in the bearing stack as a result of the interference fit. The end cap is then installed to apply a predetermined clamping force to the outboard end of the bearing assembly so that the compressive load is theoretically maintained during the dynamic loading conditions in service.
The journal is the smallest diameter portion of a rail car axle, and bending loads applied through the bearing to the outwardly extending, or cantilevered journal tends to cause flexing of the journal particularly under the dynamic loading of a fast moving, heavily loaded rail car. As explained in U.S. Pat. No. 5,380,102, the cantilevered loading of the rotating journal results in dynamic flexing, i.e., the journal is always bent downward as it rotates. This flexing produces a stress in the journal which typically is at a maximum at a location just slightly axially inboard of the smallest diameter portion of the fillet, or at about the location of the axially inboard end of the conventional inner wear ring.
Flexing of the journal, particularly where adequate clamping force of the bearing is not achieved or maintained, will result in the bearing experiencing face wear between adjacent components, especially on the side of the bearing nearest the wheel where journal stresses are greatest. Under conditions of high dynamic loading, flexure can result in bearing components moving vertically relative to each other or separating on the side of the journal under tension. Fretting may also occur between the journal outer surface and the bearing cones, or the wear rings, particularly in the area where the axially inner cone face and wear ring abut. This fretting can ultimately lead to costly journal repairs or even axle replacement.
Numerous attempts have been made to reduce journal flexing and thereby damage due to fretting. For example, in U.S. Pat. No. 5,462,367, the length of the journal is reduced by eliminating the conventional wear rings and thereby reducing the bending moment on the journal. In this patent, the inner bearing cones are extended axially to provide cylindrical sealing surfaces and radial abutment faces which abut directly against the end cap and against the face of the backing ring at substantially the smallest diameter portion of the fillet. The bearing cones are undercut in this extended portion and this is alleged to reduce fretting between the inner cone and the journal surface at this high stress location. While the shortened lever arm of the resultant load inherently reduces journal flexure, and thereby presumably reduces fretting, this bearing design may present additional problems. For example, the inner races of this bearing are not interchangeable with the millions of rail car bearings currently in use so that interchangeability of parts is not possible. Further, the bearing seal, extending directly between the bearing cup in the relatively large diameter extended portion of the bearing cones, provides a lubricant reservoir at the ends of the rolling elements which is extremely small and may be difficult to seal. The tapered rollers, particularly at high speed, inherently act as a pump tending to force lubricant from the central portion of the bearing cavity outwardly to the sealed end portions from which the lubricant must migrate back toward the central portion of the cavity. The reduced reservoir at the outer ends of the bearings can thus result in excessive pressure build up, and seal leakage and may result in improper bearing lubrication. Also, the abutment faces between the inboard bearing cone and the backing ring is located substantially at the point of maximum stress. This can result in end face fretting which, in time, can relieve bearing stack preload.